1. Field of the Invention
The present invention generally relates to a magnetic tape cassette. It particularly relates to an compact magnetic tape cassette whose size is not larger than a magnetic tape cassette of the Philips type and which is used in audio equipment or the like.
2. Background of the Invention
In recent years, the size and weight of cassette tape recorders has been reduced. A compact magnetic tape cassette has become widely used for cassette tape recorders of reduced size and weight. In the field of audio equipment, magnetic tape cassettes usable for recording and reproduction of high quality and density have been long desired. A type of magnetic tape cassette which meets this desire is is the magnetic tape cassette of the digital type which records pulse code modulated (PCM) signals. In this system, the input signal is converted into pulses for recording and reproduction in an operation different from the analog type.
Nowadays, large video tape cassettes have become generally used for digital recording. In this large video tape cassette, the recordable frequency band is made about five times wider than that for a conventional audio tape cassette. The large video tape cassette is recorded and played with a rotary-head system (except for special cases) and has such a construction that an opening provided in the front of the cassette covered by a guard panel openable upward from the cassette. Since the large video tape cassette is of the digital type, recording and reproduction of high density and wide frequency band are performed with a shorter recording wavelength than in a conventional compact audio tape cassette. For that reason, the video tape cassette needs a design that includes dust-proofing and tape protection measures.
The magnetic tape cassette, to which the present invention relates and which meets the above-mentioned desire, can be used for audio equipment or the like for which a recording/reproduction system of relatively wide frequency band is adopted and can be used as well as a video tape cassette. The size of the magnetic tape cassette is no larger than that of the conventional compact audio tape cassette and is therefore very small.
A mechanism has been proposed which detects the end of a magnetic tape to stop the operation of a recording/reproduction machine at the end of movement of the tape when recording or reproduction is performed using a magnetic tape cassette. In one such mechanism, a light emission element and a light reception element are disposed on either side apart of the tape movement path to determine whether or not light from the light emission element reaches the light reception element. If the light is transmitted through a transparent tape provided at the end of the magnetic tape, the end of the magnetic tape is detected. Since the light emission element and the light reception element are provided in the recording/reproduction machine, there remains extra internal space in the magnetic tape cassette if the size of the cassette is as large as that of a conventional video tape cassette. In that case, one of the light emission and light reception elements can be put in the magnetic tape cassette and located behind the magnetic tape. However, since the size of the magnetic tape cassette to which the present invention relates is so small that it is very difficult to put the light emission element or the light reception element in the cassette, both the light emission element and the light reception element need to be placed outside the cassette.
A magnetic tape cassette has been proposed, as shown in FIG. 1, which is constructed so that an L-shaped prism 1 is provided in the cassette and light from a light emission element 8 is reflected behind a magnetic tape 10 and then reflected again to reach a light reception element 9 in a path passing through a tape movement path. The prism 1 is made of a transparent material and comprises a horizontal portion 3 having a light reception surface 2 and a vertical portion 4. The top and bottom of the vertical portion 4 are provided with a first reflection surface 5 and a second reflection surface 6. The prisms 1 are secured inside both the side walls of the cassette, for example, so that the light reception surface 2 and the second reflection surface 6 face outward with regard to the cassette. The side walls corresponding to the light reception surface 2 and the second reflection surface 6 are provided with notches or through holes. The magnetic tape 7 is moved in the space between the vertical portion 4 and the side wall of the cassette. The recording/reproduction machine includes the light emission element 8 opposed to the light reception surface 2 of the prism 1 and the light reception element 9 opposed to the second reflection surface 6 of the prism across the magnetic type 7. The light from the light emission 8 enters into the prism 1. When the magnetic tape 7 is being moved over the horizontal portion 3 of the prism 1, the light from the second reflection surface 6 does not reach the light reception element 9 but is blocked by the opaque magnetic tape 7. When a transparent tape 10 continuous with the end of the magnetic tape 7 is being moved, the light from the second reflection surface 6 reaches the light reception element 9 through the transparent tape 10. The end of the magnetic tape 7 can be detected by finding out whether or not the light entering the prism 1 from the light emission element 8 reaches the light reception element 9. However, the larger the angle of incidence of the light to the light reception surface, the more the light is reflected by the surface 2. The amount of light entering the prism 1 thus decreases, and the light reflected by the surface 2 reaches the light reception element 9 directly, to cause the magnetic tape end detection mechanism to operate improperly.
Since the size of the magnetic tape cassette to which the present invention relates is small and the distance between the light emission element and light reception element of a recording/reproduction machine for the cassette is short, the light reflected by the light reception surface is more likely to reach the light reception element. Therefore, this is a very serious problem.